It is known to mount the two reels of such an apparatus on a pair of coaxially juxtaposed mandrels which are disposed for independent rotation on a shaft which is cantilevered on a swingable arm or other support containing a transmission for driving at least the take-up reel, usually through a slipping clutch. With a recording medium such as a tape or a film advanced at constant speed between these reels, the speeds of the supply and take-up reels progressively increase and decrease, respectively, on account of the changing radii. Thus, the two adjacent reels generally turn at different velocities and must therefore be held spaced apart in order to prevent the supply reel from being frictionally retarded or accelerated by the take-up reel with resulting overstretching or excessive ballooning of the recording medium.
In order to insure such a separation between the two reels, it has already been proposed to make the diameter of the inner mandrel (i.e. the one closer to the supporting bar) larger than that of the outer mandrel (i.e. the one near the free shaft end), either on manufacture or with the aid of an adapter slipped over the inner mandrel. This requires, of course, that the central bores of the two reels also differ in their diameters, with the result that these reels cannot be used interchangeably.